Platforms used in offshore petroleum production have traditionally been designed to resist wind, wave, and current environmental forces in a rigid, generally immobile, manner. In contrast, the relatively new tension leg platform concept (hereinafter referred to as the "TLP") is an offshore structure designed to react to environmental forces in a compliant, responsive manner. Characteristically, a TLP's main body, the "hull", is buoyant and floats on the water's surface. The hull is anchored to foundation units on the ocean floor by a set of substantially vertical tethers. The tethers maintain the hull in position above the subsea wells and partially restrain its response to the environmental forces.
The length of each tether is precisely determined to ensure that the hull floats at a somewhat deeper draft than if the TLP were unrestrained. As a result, the hull's buoyancy exerts an upward load on the tethers, thereby placing the tethers in tension. When the hull is acted upon by the environment, its compliant response affects its buoyancy, which in turn affects the tension in the tethers. The resulting cyclic variations in the tension and bending stresses in the tethers is an important consideration in TLP design.
The key analytic issue which arises from these stress variations is tether fatigue life. Generally speaking, both larger amplitude stress cycles and frequent repetitions of cycles of all amplitudes lower the expected life of a tether. Optimum tether design focuses on minimizing these undesirable occurrences.
A typical TLP has one or more tethers, which are typically made of elongated tubing sections, at each corner of its hull. The sections are joined to each other by mechanical connectors or by welding. As offshore petroleum production operations progress into deeper waters, connecting adjacent sections of tubing becomes increasingly difficult. Therefore, optimum TLP design utilizes as few tethers as possible both to minimize tether cost and weight and to reduce installation complications. However, the use of fewer tethers necessitates tether designs with large diameters and thick walls, each of which tend to reduce tether fatigue life. Alternate tether designs are needed to lessen the TLP designers' constraints between cost, weight, and installation considerations on one hand and fatigue life on the other hand.
Tether design is also sensitive to water depth. As water depth increases, longer tethers are required. With increasing length, tethers are exposed to more severe resonant loading, which is the response to a periodic driving force (e.g., waves and/or winds) which has a frequency approximately equal to the natural undamped frequency of the tether. That resonant response increases the risk of damage to or loss of the tether. Loss of a tether would increase loads on the remaining tethers, thereby posing a substantial risk of damage to the entire TLP. Alternate tether designs are needed to lessen or eliminate that risk.
For these reasons, therefore, a need exists for a TLP tether which minimizes the necessity for connecting numerous segments of tubular piping, which reduces the fatigue life constraints placed on the designer by existing tether designs, and which reduces the overall weight of the tethers.